AI when embarking a vehicle will open and close the door to the vehicle each time no matter how many of them are boarding.
For example you order 3 soldiers to board a transport UH1: each soldier will open the cabin door, get in, and close the cabin door. Same for getting out. This creates ludacris delays in disembarking vehicles when obviously the first guy would open the door and the last guy would close it.
This applies to most vehicles with a door.